The physical structure of the seven human herpesvirus virions is remarkably similar in all herpesviruses. The internal capsid is composed of seven viral proteins and is dependent upon another six viral proteins for efficient encapsidation of DNA. The long-term objectives are to understand the protein-protein interactions required for packaging DNA and resolve them into distinct reactions in vitro. The herpes simplex virus type 1 (HSV-1) UL28 protein is the most interesting of the proteins required for encapsidation and appears conserved among every avian and mammalian herpesvirus. The Specific Aims of this proposal will elucidate the function of UL28, determine the equivalence of this function among herpesviruses, identify the protein which associates with UL28, and elucidate individual steps involved in capsid maturation. Experiments in AIM 1 will identify the functional domains of the pseudorabies virus (PRV) UL28 protein responsible for structural integrity and function. Selected amino acids in PRV UL28 that are identical in 9 divergent herpesviruses will be mutagenized to create PRV mutants. These virus mutants will be analyzed for alterations to UL28 structure and function. These experiments will lead to a working knowledge of domains required for oligomer formation, DNA packaging, and capsid maturation. Experiments in AIM 2 will determine the compatibility of the HSV-1 and PRV UL28 proteins. The functional determination of similarities and differences among herpesviruses is essential for development of a broad-basis antiviral agent. Heterologous UL28 viruses will be constructed to demonstrate equivalence if the proteins complement each other. Type- specific and type-common domains will be mapped if these proteins are not compatible. Experiments in AIM 3 will identify the protein this stably associates with UL28 in infected cells and duplicate this interaction in vitro. An investigation of this protein and its interaction with UL28 is critical for a complete understanding of capsid maturation. This protein- protein interaction will be exploited to identify domains essential for this association. Herpesvirus proteins involved in DNA encapsidation are essential and probably independent of normal cellular functions. Therefore, these proteins are attractive targets of antiviral agents. The proposed experiments will provide the information necessary to develop anit-herpesvirus compounds specific to herpesvirus infections with minimal effects to the host.